A plug-in hybrid electric vehicle (PHEV) has an engine and an electric motor. The energy source for the engine is fuel stored in the vehicle and the energy source for the motor is a battery of the vehicle. The battery has a relatively large charge storage capacity and is primarily rechargeable from the external electric grid. As much pre-saved battery energy as possible should be used before the next battery charge event; i.e., the relatively low cost grid supplied electric energy should be fully used for vehicle propulsion and other functions prior to the next charge event. The fuel economy of a PHEV can be improved when the onboard energy (fuel and electricity) usage is optimized (i.e., adapted) for the PHEV's trip. Optimization of energy usage to a trip is defined as the energy usage which minimizes fuel consumption for the trip. As such, minimizing fuel consumption per charge cycle may be attained when a PHEV is controlled while traveling during a trip in accordance with a battery state-of-charge (SOC) depletion profile optimized for the trip.